Mycophenolate mofetil (MMF) was approved by the FDA in 1995 for the prevention of rejection in renal transplant recipients. (1-7) MMF is a prodrug which appears to be rapidly and completely hydrolyzed to mycophenolic acid (MPA) the active form of the drug on absorption. MPA is further metabolized to mycophenolic acid glucuronide (MPAG) an inactive metabolite. On going trials of MMF in combination with other immunosuppressive agents are being conducted in heart transplant patients.(8) Although pilot studies with MMF in combination with other immunosuppressive agents (cyclosporine and corticosteroids) have been conducted for the prevention of rejection in liver transplant patients (data not yet published), the proposed study will be the pivotal efficacy and safety trial for FDA approval in liver transplant patients. Five hundred and fifty liver transplant patients will take part in this randomized, double-blind, multicenter study of the efficacy and safety of intravenous and oral mycophenolate mofetil and azathioprine, each in combination with cyclosporine (Neoral) and corticosteroids. A sub-group of 30 patients will take part in the pharmacokinetic portion of this study. Twelve hour, steady-state pharmacokinetics of oral MMF will be performed at 6 months post-transplant in the CRC.